Climate Change & Sea Level Rise

One of the great challenges facing this and any other restoration project is global climate change. As described below, the two most prominent considerations for the SBSP Restoration Project are the impacts of sea level rise and the potential for mitigation of carbon emissions via carbon sequestration in tidal marsh habitats.

Impacts of Sea Level Rise

We are frequently asked how the Project incorporates sea level rise into the program-level planning, and how sea level rise would be considered in future project-level design and planning. Estimates of sea level rise typically provide a range of potential rates due to uncertainties in the prediction models and uncertainties related to future greenhouse gas emission scenarios. The Project generally utilized a mid-range sea level rise estimate for analysis1, and planned for a wider range of possibilities through phased implementation and adaptive management. More detail is provided in the following sections.

Estimates of Future Sea Level Rise

The Project’s Final EIS/R utilized the 2001 Intergovernmental Panel on Climate Change (IPCC) mid-range sea level rise estimate of 6 inches by 2050 (3 mm/yr average) and 18 inches by 2100 (6 mm/yr average between 2050 and 2100) (IPCC 2001). The higher rates in the second half of the century reflect the effects of accelerated sea level rise. The IPCC uses a consensus-based process involving many hundreds of international experts on climate change.

In February 2007, the IPCC released a “Summary for Policymakers” of the full May 2007 report “The Physical Basis of Climate Change” (IPCC 2007) just as the Draft EIS/R was being finalized.

As described in the Executive Summary of the EIS/R, the IPCC summary included revised sea level rise estimates for the twenty-first century (2000 to 2100), and the revised estimates were compared with the previous IPCC (2001) estimates used in the EIS/R. The 2007 IPCC estimates are slightly lower than the 2001 estimates with a narrower band of uncertainty (IPCC 2007). The 2001 IPCC estimate was selected because the EIS/R analyses occurred between January 2004 and February 2007 when the 2001 rates were the most recent.

It is important to note that the IPCC projections do not include the contribution of changes in ice sheet melting (referred to as ice sheet mass wasting) to sea level rise due to significant difficulties in predicting these contributions. The state of the science of sea level rise has been changing very rapidly recently. Over the last year, there have been major advances in the science, suggesting that future sea level rise may be much greater than that predicted by IPCC (2007). Semi-empirical models project a higher mid-range sea level rise of 28-39 inches (70-100 cm) over the next century and a larger range on the high end2. The Project would plan, design, and manage for higher rates of sea level rise using phased implementation, monitoring, and adaptive management, described below and in the EIS/R.

Sea Level Rise and Habitat Restoration

The consequences of accelerated sea level rise were evaluated in the habitat evolution assessment (South Bay Geomorphic Assessment [SBGA], EIS/R Appendix I). Watson (2004) showed that the high sediment availability in the far South Bay sustained marshes at a time when subsidence was very high. Therefore, if sea level rise rates match the lower to mid-range of the predictions and sediment availability remains high, tidal marshes in the South Bay should keep pace with changing conditions as they have done historically. If higher rates of sea level rise prevail, the timeframe for marsh development may be delayed, and tidally-restored areas within the SBSP Restoration Project Area may persist as intertidal unvegetated mudflats or shallow open water habitat for prolonged periods. However, the South Bay, and in particular the far South Bay, have historically been sediment-laden depositional environments (Jaffe and others 2006a, Jaffe and others 2006b), therefore the tidally-restored ponds are expected to accrete sediment and vegetation is expected to establish in the face of accelerated sea level rise (Appendix I).

It should be noted that the SBGA, which evaluated long-term trends with respect to the South Bay’s sediment budget, geomorphic change and mudflat loss, evaluated a range of sea level rise scenarios, including doubling the IPCC 2001 mid-range estimate. Doubling the IPCC 2001 mid-range estimate is consistent with the IPCC (2001) high-end estimates. In this case, a range was evaluated because sediment budgets and sediment sinks are highly sensitive to the rate of sea level rise. The SBGA and the EIS/R acknowledge that higher than anticipated rates of sea level rise could have a considerable effect on the mix of habitats within the SBSP Restoration Project Area and within San Francisco Bay in general.

Higher than anticipated sea level rise rates that result in delayed or arrested marsh establishment could affect the progression between the 50:50 and 90:10 alternatives presented in the EIS/R. Tidal habitat restoration may be closer to the 50:50 bookend to increase the sediment supply to those ponds that are tidally restored. Adaptive management efforts would be used to encourage marsh establishment in the tidal ponds. The restoration actions most sensitive to sea level rise would contain features to accommodate accelerated sea level rise, such as constructing a gradually sloping marsh/upland transition zone surface that provides an elevation gradient over which tidal marsh could shift upslope as sea level rises and initiating marsh vegetation plantings to maximize sediment-trapping efficiencies and enhance the accumulation of organic matter in the developing marsh sediments.

Sea Level Rise and Flood Protection

The future design of the flood protection levees would also take into account the best available information on sea level rise at the time of project-level planning and design. The plans would outline a strategy for low-, mid-, and high-end sea level rise predictions. For example, the plan may include building a levee to accommodate the 50-year mid-range sea level rise projection, and incorporate features or outline a process to deal with higher or lower rates of sea level rise. Lower than anticipated sea level rise is generally not anticipated to be a problem. Higher than anticipated sea level rise would require subsequent design phases to raise the levee (i.e., widening and raising the levee or building a flood wall) before sea level rises above the design level for flood protection. Other options would include overbuilding the levee initially to anticipate a higher rate of sea level rise, either by building a higher levee, or by building a levee with a wider base to more easily accommodate future increases in levee height. The future design of the flood protection levee would balance the cost and benefits of the potential approaches at the time of design. The project-level analysis and design would be presented in a future project-level EIS/R. Subsequent phases of environmental documentation may also be required to address changes to the Project based on updated sea level rise information and analysis. For example, there may be a need to import more fill than currently anticipated in this programmatic EIS/R for flood protection levee construction and maintenance of the flood protection and managed pond levees.

Phased Implementation, Monitoring, and Adaptive Management to Address Uncertainty in Future Sea Level Rise

The Project would use phased implementation, monitoring and adaptive management to plan for and accommodate a range of potential future sea level rise. Updated sea level rise estimates would be used as future phases were designed and implemented. Monitoring and adaptive management would provide updated assessments of future sea level rise, inform planning for future phases, and adjust previously- implemented phases as needed. These are described in the Adaptive Management Plan and summarized in Section 2.3 of the EIS/R. Examples of monitoring and adaptive management activities:

• As part of the adaptive management program, the Project would monitor sea level rise in the South Bay and review the scientific literature on sea level rise on an ongoing basis (discussed in EIS/R Section 2.3 and Appendix D).
• Additional monitoring and modeling of sediment dynamics within the South Bay are planned as part of the Adaptive Management Plan. A longer-term modeling effort led by Principal Investigators at U.C. Berkeley and Stanford University is being initiated to develop a coupled hydrodynamic and sediment transport model of the South Bay. The model, coupled with monitoring data from previous restorations and the Phase 1 actions, would inform future phasing and implementation with respect to sea level rise, sediment supply and sediment sinks. In the long term, the model may be extended to include morphological, water quality and biological modules to improve the ability to predict ecosystem response to restoration actions.
• The Adaptive Management Plan and SBGA provide examples of adaptive management actions that could be used to narrow the range of uncertainties and encourage restoration success: adjusting the phasing to better match the sediment supply; maintaining levees along the bayfront edge to shelter restored tidal areas from wave energy and encourage marsh formation; removing levees along the bayfront edge to restore sustainable mudflats within the ponds; restoring natural shorelines such as shell breaches, wrack lines, and Bay-edge pans; using imported fill to raise pond beds to elevations conducive to vegetation establishment; and prioritizing restoration of less subsided ponds and/or ponds close to sediment supplies within the Project Area.

In summary, the Project would seek to accommodate accelerated sea level rise, to the extent practicable, in order to maximize achievement of the Project Objectives.

Potential for Carbon Sequestration

Current research shows that restoring tidal salt marshes is one of the most effective measures for sequestering carbon available to us. While people often look to planting trees as a way to take carbon out of the atmosphere, marsh restoration may be even more efficient, per unit area, at removing carbon. Tidal marshes are extremely productive habitats that capture significant amounts of carbon from the atmosphere, which are stored in marsh soils. Unlike many freshwater wetlands, saltwater tidal marshes release only negligible amounts of methane, a powerful greenhouse gas; therefore, the carbon storage benefits of tidal salt marshes are not reduced by methane production. In addition, as sea levels rise, tidal marsh plains continue to build up to match the rise in water level—if suspended sediments are adequate—continually pulling carbon dioxide out of the air in the process. While specific research is needed to quantify the carbon sequestration capacity of San Francisco Bay tidal marshes, in general, restoring tidal marshes is an effective method, recommended by the Intergovernmental Panel on Climate Change, for removing carbon dioxide from the atmosphere. Researchers Choi, et al. (2004) conclude that, “Because of higher rates of C sequestration and lower CH4 emissions, coastal wetlands could be more valuable C sinks per unit area than other ecosystem in a warmer world.” Read more on this issue in the White Paper on Carbon Sequestration and Tidal Salt Marsh Restoration (Trulio, Callaway, and Crooks 2007).